Sodium percarbonate is used as a bleaching agent and as an active bleaching component in detergents and cleaning agents. The disadvantage of sodium percarbonate here is that it tends to decompose in contact with moisture and with components of the detergent and cleaning agent compositions, leading to a loss in active oxygen and hence in bleaching action. In these applications sodium percarbonate is therefore preferably used in the form of granules coated with a stabilising coating layer in order to achieve improved storage stability in detergents and cleaning agents.
It is known from EP-A 0 863 842 that sodium percarbonate granules with improved storage stability can be produced by applying a firmly adhering coating layer, consisting substantially of sodium sulfate, to a core of sodium percarbonate produced by fluidised bed spray granulation.
WO 96/06801 describes coated sodium percarbonate granules having improved stability that are synthesised from a core of sodium percarbonate granules, an inner coating layer containing a sulfate or silicate and an outer coating layer containing a compound (B) selected from salts of monocarboxylic and dicarboxylic acids having 4 or more carbon atoms and a compound (C) selected from salts of sulfated oils, α-olefin sulfonic acids and monoalkyl and dialkyl sulfosuccinates. The examples given in the specification show that the presence of compounds (B) is necessary to achieve an adequate storage stability and that granules containing only compound (C) and no compound (B) in the outer layer display unsatisfactory storage stability.
JP-A 06-263434 describes stable in storage sodium percarbonate granules that are synthesised from surfactant-containing sodium percarbonate granules as the core, an inner coating layer consisting of a sulfate and an outer coating layer consisting of an anionic surfactant. Here the core material is produced by crystallisation and subsequent granulation with addition of the surfactant. From the examples and comparative examples given in the specification it follows that the core material must be produced with addition of a surfactant in order for the coated sodium percarbonate obtained to display an adequate storage stability and that granules with a surfactant-free sodium percarbonate core, an inner coating layer consisting of a sulfate and an outer coating layer consisting of an anionic surfactant display insufficient storage stability.
In the case of the sodium percarbonate granules described in EP-A 0 863 842 there is still a need for an improvement in storage stability. The sodium percarbonate granules described in WO 96/06801 have the disadvantage that they are complicated to produce, since at least three different compounds are necessary for coating the sodium percarbonate granules.
The sodium percarbonate granules described in JP-A 06-263434 have the disadvantage that with the surfactant they contain an organic compound in the sodium percarbonate core. This has a disadvantageous effect on the internal stability of the sodium percarbonate granules and leads to restrictions in the storage of large amounts of the granules in silos. For storage in silos the sodium percarbonate granules must display high internal stability, in other words low heat release during storage, since otherwise decomposition of the sodium percarbonate granules can occur in the silo due to self-accelerating heating.